Duct leakage control

ABSTRACT

The invention relates to the control of leakage from ducts, such as fluid-carrying pipework, without necessarily requiring direct access to the leakage site itself; since leakage sites are often difficult to locate with the necessary degree of precision to permit access to reliably be made thereto, and moreover, even if a site can be located, it is not always convenient or economically possible to secure access thereto. The invention provides an apparatus and methods for duct leakage control wherein a sealing element ( 1 ) is introduced into the duct and is automatically drawn or otherwise guided to the locality of a leak; the element being caused, by reason of a pressure differential attributable to the leak, to move into and stem or seal the leak. The sealing element may comprise a plurality of individual members of differing buoyancy, each capable of being carried along at a predetermined level in the duct by the flow of fluid therein. The sealing element may carry a tagging device which can be used to assist in locating the leakage site.

[0001] This invention relates to the control of leakage from ducts, suchas fluid-carrying pipework, and aims to provide means to reduce orprevent leakage from apertures compromising the integrity of ducts andattributable to causes such as manufacturing faults or blemishes, rustor other corrosive activity, piercing damage, age of pipework, orjoints.

[0002] It is usually the case, for one reason or another, thatsignificant areas of fluid-carrying ducts, once installed as part of apipework system, become substantially inaccessible. For example, mainswater distribution systems employ vast lengths of buried pipework whichinvolves expensive and time-consuming excavation to expose, for scrutinyand/or repair, areas of pipe from which leakage is suspected.

[0003] An efficient technique for locating leakage without resort tomajor excavation is thus required, and such a technique is described,for example, in International Patent Application No. PCT/GB 99/03742.

[0004] That application describes a technique wherein a sensor means isprovided within the pipe for detecting characteristics of the fluid. Thedetected characteristics are recorded and used to evaluate a fluid flowfield characteristic which is compared with a reference fluid flow fieldcharacteristic for that pipe to obtain thereby data concerning a leak.The comparison may usefully be effected by means including a neuralnetwork.

[0005] It is thus known that leakage can be localised without excavationor other expensive investigative techniques.

[0006] However, it may still be necessary to excavate or otherwiseuncover the leaking pipework in order to control the leak, and thepresent invention seeks to reduce or eliminate the need for suchactivities.

[0007] Moreover, there are occasions, particularly in the oil, gas andnuclear industries, when a rapid response to leakage (or to a lack ofcontainment generally) is essential. In addition there is often theneed, having stemmed the initial flow of fluid from a leak, toaccurately locate the leak so that a permanent repair may be made, orother remedial action taken. Still further, there may be a need in someoperational circumstances to shield or protect workers, engaged in thelocation of leaks and/or the associated repairs, from the leaking fluid.

[0008] Certain embodiments of the invention are intended to address atleast one of the foregoing requirements.

[0009] According to the invention, there is provided a method, apparatusor system for duct leakage control wherein a sealing element isintroduced into the duct and is automatically drawn or otherwise guidedto the locality of a leak and is caused, by reason of a pressuredifferential at that locality and attributable to the leak, to move intoand stem or seal the leak.

[0010] In one embodiment, the sealing element comprises a plurality ofindividual members, preferably of differing buoyancy, each capable ofbeing carried along by the flow of fluid along the duct; one or more ofsuch members being attracted to the proximity of a leak by pressuredifferential attributable to the leak, and being so shaped andconstructed that, once drawn against the inside face of the duct in thevicinity of the leak, it acts to seal the leak, or at least to stem theflow of fluid therethrough. The members may be of differing sizes andshapes, and may be selected so that one or more secondary elements canact in concert with a primary element already in place, and stemming theflow of fluid through a leak, to effect a complete seal.

[0011] Systems utilising such elements may or may not require thelocation of a leak to have been previously determined by means such asthose described in the aforesaid International Patent Application.

[0012] In another embodiment of the invention, the technique describedin the aforesaid application is used to locate the leak, and its sensormeans may be used to either transport or guide the sealing member to theleak. In this embodiment, the sealing member may, for example, take theform of an open-ended frustoconical sheet, somewhat like a wind sock inappearance, and it may be towed by the sensor of the leak-locatingsystem or attached to another member which is guided by the sensor tothe vicinity of the leak, whereupon the sheet is drawn towards the leakby the pressure differential thereacross. Preferably the sealing membersupports, in the form of a coating or otherwise, a medium capable, whenforced into contact with the inside wall of the duct, of adheringstrongly thereto, thereby anchoring the sheet firmly in place across theleaking area of the duct wall and sealing the leak.

[0013] Clearly, unless the sensor is actually used to tow or otherwiseconvey the sealing member and deploy it at the scene of the leak, it isnot necessary for the sensor to be in place in the duct at the time thesealing is effected. As long as the sensor has done its work in locatingthe leak, and the leak's position is thus known to the system operators,the sealing means may be guided to the identified location by meansoperated remotely from the duct.

[0014] In other preferred embodiments, the sealing means may be providedwith a device capable (either by itself or in co-operation with anotherelement or component) of signalling the location of the sealing means,at least from such time that the sealing means becomes stationary, in aposition sealing a leak.

[0015] The device may be a passive device such as an antenna loop orsimilar, the proximity of which can be sensed by a mobile intelligentunit inside the pipe or a suitable external pick-up. Alternatively, anactive device such as an infra-red, acoustic, radio or optical sendermay relay signals either directly to the environment outside of the pipeor to a mobile intelligent unit inside the pipe.

[0016] In order that the invention may be clearly understood and readilycarried into effect, embodiments thereof will now be described, by wayof example only, with reference to the accompanying drawings, of which:

[0017]FIG. 1 shows a sealing element for use in a first embodiment ofthe invention;

[0018]FIG. 2 shows schematically the element of FIG. 1 in place to sealor stem a leak;

[0019] FIGS. 3(a) and 3(b) show sealing elements for use in a secondembodiment of the invention;

[0020]FIG. 4 shows schematically a sealing element of the kind shown inFIGS. 3(a) and 3(b) in place to seal or stem a leak.

[0021]FIG. 5 shows schematically apparatus for use in the method of thepresent invention;

[0022]FIG. 6 shows a relationship between the apparatus of FIG. 5 whenprovided in a pipe with a leak and a measurement of flow field;

[0023]FIG. 7 illustrates an array of differential sensors for use in theapparatus shown in FIG. 5;

[0024]FIG. 8 illustrates one example of detected pressurecharacteristics of a leak using the sensor array of FIG. 7; and

[0025]FIG. 9 illustrates schematically, and cross-sectionally of a pipe,a composite sealing element for use in certain embodiments of theinvention.

[0026] The present invention can be applied to fluids generally, forexample, oil, water, natural gas etc. The present embodiment will bedescribed with respect to water.

[0027] As a fluid passes through pipework, it can be represented as aflow field varying according to the spatial location within the pipe.The characteristics of the fluid flow field will vary according to alarge number of parameters, including for example pipe size (diameter),fluid pressure, pipe surface characteristics, the type of flow, sidepassages, directional variation of the pipe etc. Another parameter thatwill modify the fluid flow field characteristics is the presence of aleak. Indeed, the degree of modification of the fluid flow fieldcharacteristics will vary according to the form of leak, for example itssize, type, geometry and location within the pipe.

[0028] However, it has been found that one of the particular problems indetecting a leak is that the effects of the leak on the fluid flow fieldare very difficult to detect. There are a number of reasons for this.One reason is that the overall flow of the water in the pipe is notlaminar. Instead, there is a continual background turbulence within theoverall flow. This tends to mask the modifications to the fluid flowfield resulting from the leak. Another reason is that the modificationsto the fluid flow field resulting from the/leak are extremely small andhighly localised. For example, the localised drop in pressure relativeto the gross or ambient pressure in the pipe that results from a leak ofdiameter a is of the order of ¼% with the effect of the leakdisappearing within a pipe length of 5a to 10a before and after theleak. This makes it extremely difficult to detect a leak at all and inparticular to pin point the location of a leak.

[0029] The inventor considers that a part of the reason for the verysmall localised effect of a leak is that the flow within the piperecovers very quickly after the leak. In fact, the disturbance in theflow field characteristic probably derives from a velocity component ofthe leaking water as it flows through the leak, this velocity componentbeing essentially normal to the direction of the gross flow of water inthe pipe. The effect of this component will be small within the terms ofthe gross water flow and will disappear rapidly either side of the leak.

[0030] By varying the aforementioned parameters with assorted forms ofleaks whilst monitoring the characteristics of the fluid flow field, andby using various processing techniques, it is possible to correlate leakfrom with fluid flow field characteristics thereby enabling the locationof leaks and determination of leak form.

[0031] Referring to FIG. 5, a leak location system comprises a sensormeans, in the form of a capsule 2, which is located in a pipe section 1through which water is flowing in the direction of the arrow. The pipesection has preset parameters. The capsule has dispersed around itsperiphery a plurality of sensors 3 for measuring a fluid flow fieldcharacteristic within the pipe interior. Measurements taken by thesensors 3 are communicated to a remote computer 4. In this respect, themeasurements may be communicated in real time, for example by way of atransmitter on the capsule and a suitable receiver for the computer.Alternatively, the measurements may be stored in a memory on thecapsule, for example on a smart card, the data from which can betransferred to the computer following a passage of the capsule withinthe section of pipe.

[0032] The capsule 2 is arranged to traverse along the interior of thepipe in the flow of water. In this respect, the capsule may rely on theflow of water to carry it through a designated section of pipe, or itmay be provided with propulsion means for affording it independentmovement within the flow. The capsule is provided with location meanswhereby the location of the capsule within the pipe section can bedetermined. The location means may comprise, for example, ultrasonicsensors for accurately measuring distances to pipe walls arid/or GSMtechnology for accurately determining the position of the sensor withine.g. a long stretch of pipe.

[0033] A movement control means can also be provided for adjusting thespatial position of the capsule. Such movement control means may includeradio controlled vanes or fins or the like for guiding the capsulewithin the flow.

[0034] The data from the measurements of the sensors are processed by aprocessing means and stored in memory. A series of data are obtainedfrom pipe sections having different parameters and with and withoutleaks of varying forms whereby a library of fluid flow fieldcharacteristics is built up which can be differentiated according tothese parameters. By using artificial neural networks and repeating thetaking of data a large number of times, the artificial neural networklearns to classify the fluid flow field characteristics with more andmore accuracy.

[0035] The use of the above described system is now described inrelation to FIG. 6.

[0036] In order to examine a section of pipe, a suitable inlet in thepipe is required for insertion of the capsule into the flow. Onceinserted, the capsule 2 traverses along the pipe taking separatepressure measurements from the individual pressure sensors 3. Suchmeasurements are recorded within the capsule. The fluid flow fieldcharacteristics as represented by the pressure measurements are modifiedby the presence of a leak as shown by the pressure contour 6. Hence, thepressure measurements taken by the sensors on the capsule provideinformation about the presence of the leak according to the graphicalplot of pressure in relation to distance along the pipe section, asshown in the figure. A leak 5 is positioned at the point x=L along thelength of the pipe, the plot showing this to be an area of pressurefluctuation. The plot is only illustrative and will vary depending onthe geometry of the leak.

[0037] When analysing the measurements, details of the pipe geometry canbe entered in the computer 4. In this manner, the relevance of amodification of the fluid flow field characteristic can be moreaccurately considered.

[0038] Thus, the measurements taken by the capsule are not necessarilyparticularly helpful on their own in establishing conditions within thepipe, e.g. the position and geometry of the leak. In order to interpretthe measurements in detail, they are compared with similar measurementsfrom other pipe sections having known internal conditions. Such knownmeasurements may be held by the computer in a library of priormeasurement and associated pipe internal condition data. In this regard,the measurements can be used to form a fluid flow field signature of thepipe under examination as shown generally by the plot in FIG. 6. Whensuitably categorised or classified, such a signature can be readilycompared with signatures formed for prior known pipe internalconfigurations to draw out relevant characteristics.

[0039] As such, the measurements themselves do not need to be fullyunderstood, merely compared with previous measurement data having knownassociated pipe characteristics.

[0040] In carrying out the analysis of the measurements, neural networksmay be employed. Pre-processing of the measurements can be conductedusing several techniques such as principal component analysis, wavelettransforms, and higher order spectral analysis. With use ofpre-processing of the signal, it is possible to extract the maximumamount of detail about pertinent aspects of the signatures whilstminimising unwanted information and noise.

[0041]FIG. 7 illustrates an array of sensors which provides a moresensitive form of sensor means that can be applied to the capsule 2shown in FIG. 1. In particular, four differential pressure sensors (e.g.Honeywell 24 PCA) are provided in the capsule to detect the differentialpressure between four sensor outlets or tapping points A to Ddistributed radially around the body of the capsule. Thus, thedifferential pressure between outlets A-B, B-C, C-D and D-A can bedetected. Since absolute pressures are not detected, the sensors canresolve small localised pressure differentials enabling the detection ofthe small pressure drops resulting from leaks. Furthermore, due to theangular arrangement of the outlets, spatial location of the leak isprovided.

[0042] With regard to the term pressure differential, it will beappreciated that the pressure will depend on the depth of the outlet inthe water. For example, each 10 mm of depth represents 100 Pa. Thus, thesensors can easily detect a differential pressure between the outlets inthe presence of a leak.

[0043] The pressure field in the vicinity of the leak can be describedby the equation:${\Delta \quad P} = {\frac{1}{4}{\left( \frac{a}{r} \right)^{4}\left\lbrack {p_{0} - p_{a}} \right\rbrack}}$

[0044] where

[0045] ΔP is the magnitude of the pressure variation due to the leak

[0046] a is the length scale at the leak (e.g. the radius for a circularhole)

[0047] r is the distance from the leak

[0048] p_(o) is the ambient pressure in the pipe

[0049] p_(a) is the pressure outside the pipe (this could be close toatmospheric

[0050] Thus, for instance, if p_(o)=10 bar=10⁶ Pa, p_(a)=1 bar=10⁵ Pa,a=0.01 m, r=0.05 m, then ΔP=360 Pa.

[0051]FIG. 8 illustrates an example of detected fluid flowcharacteristics in the form of pressure. In particular, this figureshows results from a pipe of 0.2 m diameter with a water flow ratetherethrough of 20 l/s at a pressure of 1 bar (10⁵ Pa). A hole ofdiameter 0.01 m has been made in the pipe at a specific locationproviding a leakage of 0.7 l/s. The capsule 2 is moved incrementallyalong the pipe to take readings. In this figure, the increments are 0.1m. The capsule is moved incrementally in this case so that localisedturbulence caused by the capsule subsides to provide more stablereadings. However, the size of the increment can be reduced and byapplication of suitable pre-processing techniques as mentioned above,the instability of the readings resulting from localised turbulence canbe filtered out to provide a cleaner overall characteristic orsignature.

[0052] It can be seen from FIG. 8 that a comparison of the detectedcharacteristics for the differential detector for outlets A-B and thedifferential detector for outlets D-A shows a marked effect around thelocation of the leak. Indeed, from a point some 0.1 m upstream of theleak, the differential pressure decreases to a minimum at the leak andafter the leak is passed the differential pressure recovers within about0.03 m. The drop in differential pressure is almost 50% at the minimum.

[0053] The difference in differential pressure between the lines forsensors A-B and A-D in FIG. 5 could either be an intrinsic instrumentoffset or a relative tilt of the sensors.

[0054] It has been found that the magnitude of the leak is related tothe pressure drop measured by the differential pressure sensors.Moreover, it will be noted that since the outlets D-A are remote fromthe position of the leak, no differential pressure drop is detected.This further illustrates the localised nature of the disturbance in thefluid field characteristic. Whilst the detected magnitude of thepressure drop is related to the magnitude of the leak, it is apparentthat the further from the leak, the smaller the magnitude of thepressure drop. Nevertheless, by using the differential pressure fromopenings B-C (not shown in FIG. 4 for convenience), it is possible tostill resolve out a pressure drop that is representative of themagnitude of the leak.

[0055] Whilst the results of FIG. 8 show a simple pressure drop fortapping points A-B that is compared with the reference pressure drop fortapping points D-A, it will be appreciated that by using the analyticaltechniques mentioned above, greater accuracy in leak-detection andinformation about the leak in relation to specific pipework can beobtained.

[0056] Referring now to FIG. 1, there is shown a sealing element 1 inthe form of a flexible membrane comprising a sheet of plastics or othersuitable material having one pair of opposing sides joined together, andbeing cut so as to present open ends 2 and 3 and to adopt afrustoconical form when filled with the fluid in a pipe (not shown inFIG. 1).

[0057] In this embodiment of the invention, it is assumed that thelocation of a leak is known as a result, for example, of the use of thesensor described with reference to FIGS. 5 to 8 and as described in theaforesaid International Patent Application and the sealing element 1 isthus intended to be transported to the area of the leak, for example bybeing towed through the subject pipework behind the sensor actually usedto locate the leak, or behind another device that is guided to therelevant area, either by the sensor itself or by remote transmittermeans.

[0058] When the sealing element 1 has been transported sufficientlyclose to the vicinity of the leak, the pressure differential associatedwith the leak, which, in a mains water pipe can be as much as 16 bar, iseffective to draw the element 1 into the leakage aperture itself, asshown in FIG. 2, and the doubled sheet is thus pulled against the innerwall 4 of the pipe, shown in part at 5. This effectively seals, or atleast stems, the leak. Preferably the sheet of the sealing element 1carries a suitable bonding agent which enables the element to be firmlyadherent to the inner wall 4. The bonding agent may be contained inmicrocapsules which burst in response to the pressure imparted to theelement 1 when it is in position to seal the leak.

[0059] Pressure sensitive release means may be incorporated in thetowing link which enable the towing vehicle to leave the sealing elementbehind at the site of the leak and proceed alone to a suitablecollection point. Alternatively, the towing vehicle (assuming that theleak sensor itself is not used for that purpose) may be regarded asdisposable and remains attached to the sealing element with any motivepower disabled by remote control.

[0060] Instead of using a sealing element of the “windsock“ shape shownin FIG. 1, one or more simple streamers can be used if preferred. Ifstreamers are used, there may usefully be several, these having varyingdegrees of positive and negative buoyancy, as well as some of neutralbuoyancy which can drift from side to side. By this means, there isprovided an increased probability that one or more streamers will bepositioned within the pipe so as to be “captured” by the pressuredifferential associated with the leak, and thus able to move into theleaking area and effect the desired sealing or stemming.

[0061] In any event, once the sealing element or elements have beendeployed to seal or stem the leak, it is envisaged that the leak sensordevice will be re-employed to investigate the extent to which the actionhas been successful. If the action has not been successful, and/or hassucceeded only to a degree, a follow-up operation may be carried out,using any of the procedures described above. In the event that afollow-up operation is required to improve or complete the sealing, thensealing elements of different dimensions and/or of different materialsto those used in the first operation may be employed. Moreover, sealingelements used in a follow-up operation may carry a different bondingmedium to that carried by the element(s) used in the original operation,bearing in mind that they will need to bond primarily to the material ofthe element(s) already in place, rather than to the wall 4 of the tube5. In some cases, especially where the degree of leakage remaining aftera first operation is relatively small, the element(s) used in afollow-up operation may not be required to carry any bonding material atall, especially if the material of the elements exhibits a degree ofsurface roughness that provides frictional inter-engagement.

[0062] It is possible for sealing elements of the kind shown in FIG. 1,or for that matter for streamers of the kind described above, to beintroduced into pipework without any knowledge of the location of a leakand/or without the use of towing or steerage devices, thereby to becarried along the pipe by the flow of fluid therein and attracted to thesite of a leak by the aforementioned pressure differential. If thisapproach is to be adopted, however, it is preferred that sealingelements of the kind shown at 6 and 11 in FIG. 3 are employed.

[0063] Referring now to FIG. 3(a), and in accordance with a secondembodiment of the invention, freely mobile elements, such as 6, offlexible membrane are released into a pipe 7, part of the bottom wall ofwhich is shown at 8. The surface of wall 8 is shown as being roughenedby the presence of sediment, scaling and/or other deposits.

[0064] The elements such as 6 resemble tadpoles, and typically take theform of small streamers or flow socks. In this example, each elementsuch as 6 is provided with one or more floats such as 9 and 10.Preferably the various elements such as 6 are provided with respectivepairs of floats having different buoyancy characteristics, so thatdifferent elements will be borne at differing heights within the pipe,thereby to increase the probability that wherever, around thecircumference of a pipe, a leak may exist, one or more elements is orare disposed and presented so as to be captured by the localisedpressure differential attributable to the leak and pulled into sealingrelationship with the leakage aperture.

[0065] In FIG. 3(a), the element 6 is provided with a relatively largefloat 9 of negative buoyancy and a relatively small float 10 of positivebuoyancy. In these circumstances, the net buoyancy is negative, so theelement 6 tends to bump along the bottom wall 8, driven by the fluidflow. The small, positively buoyant float 10, however, allows theflow-based inertial forces within the pipe to move the element 6 if ithas become trapped in the roughness on the pipe wall. Once near a leak,the differential pressure forces will dominate over the inertial forcesand so the element (provided it is suitably dimensioned) will be drawninto position to seal the leak, and will be held there. As before, asuitable bonding agent may be borne by the element 6 to ensure its firmadhesion at the desired location. Usefully, elements such as 6 ofdiffering sizes may be introduced into the pipe 7; and in somecircumstances it is preferred to introduce the elements in size groups,starting with the largest. This strategy reduces the risk of smallelements being drawn out through a leakage aperture and thus lost to thesystem.

[0066]FIG. 3(b) shows another tadpole-like element 11 which is inverselyconstructed as regards buoyancy to the element 6, and is thus caused tobump along the top wall 12 of the pipe 7. The element 11 has arelatively large positively buoyant float 13 and a relatively smallnegatively buoyant float 14, but operates in a similar way to the float6.

[0067] In operation, a large number of elements such as 6 and 11, andincluding others, as aforesaid, with differing buoyancy characteristics,is released into the fluid flow upstream of a zone of pipework to betreated. These elements travel downstream with the fluid flow and one ormore of them will be attracted to and seal, or at least stem, any leaksin that zone. This is illustrated-schematically in FIG. 4.

[0068] Those elements which are not captured by a pressure differentialassociated with a leak travel further downstream and, if necessary, arecollected by means of a net or other suitable trap installed across thepipe at a convenient access location, such as a valve access point.

[0069] In order to deal effectively with leaks located at one side orthe other (at around mid-height) in some types of pipework, it can beadvantageous to link together two or more sealing devices of equal orpredetermined relative buoyancy; the linked devices thus beingencouraged to flow at a selected position within the pipe, depending inpart upon the length of the link. The linking connection may be soconstructed as to dissolve or change in length during exposure, over aprescribed period of time, to the fluid content of the pipe.

[0070]FIG. 9 shows that the equilibrium position for the sealingelements 90, 91 would be at the mid-height of the pipe 1 as the elementsare of equal buoyancy and, because of the length of the linkingconnection 92, they cannot position themselves in any other arrangement.Likewise, positively buoyant sealing elements linked with a connectionshorter than connection 92 will naturally float closer to the top of thepipe 1.

[0071] If the connection such as 92 is such as to be dissolved by thefluid flowing in the pipe, then in the event that one of the elements 90or 91 is attracted to a leak, the other will eventually become detachedand be carried away, thereby removing any tendency for the flow of fluidpast the linked elements to pull the sealing element out of the leak towhich it has become attached. If, on the other hand, the link 92 wereconstructed so as to shorten significantly, the link can be configuredto fold over, allowing the two elements 90 and 91 to bond together andjointly contribute to the seal.

[0072] It is indeed possible to utilise sealing elements of a number ofconfigurations, such as frustoconical, rectangular, tubular etc., withvarying material stiffness and some with interconnecting links as justdescribed, so as to create an interlocking ball structure. Thisstructure can, in response to a leak, contract into a flat circularcomposite which efficiently seals the leak.

[0073] The efficiency of the sealing process may be enhanced by reducingthe flow rate of fluid in the pipe zone being treated, whilstmaintaining the operative pressure. This increases the probability ofelements being entrained into the leakage aperture by the pressuredifferential associated with the leak, since that pressure, in suchcircumstances, becomes very much the dominant force acting upon therelevant elements.

[0074] In one practical test of the first-described embodiment of theinvention, a pipework loop containing water pressurised to 1.4 bar wasprovided with a leak of diameter 10 mm, giving a leakage rate of 0.7l/s. A windsock-shaped sealing element such as that shown in FIG. 1 offront diameter 80 mm; rear diameter 20 mm and length 300 mm was attachedto a sensor device, or “fish” as it is typically called, using fourlengths of cable, each 400 mm in length. When the fish was dragged pastthe leak, the windsock-shaped element was sucked to the leak and sealedit completely, reducing the leakage to zero.

[0075] When dragged further past the leak, the nose of the windsock wasfound actually to enter the leakage hole.

[0076] The performance in sealing the aforementioned leak is impressive,particularly when it is borne in mind that the pressure in standardmains water pipes can be as high as 16 bar, so the capturing of theelement by the leak and its subsequent sealing effect can be expected tobe even more effective under such conditions.

[0077] Some or all of the sealing elements are preferably constructed soas to be capable of use with a signalling system for leak and/orpositional detection purposes. Such elements (hereinafter referred to as“tagged elements”) may either transmit under their own power to a remotelocation outside of the pipe; transmit to a pipework-borne transponderwhich in turn transmits (either automatically or in response to aninterrogating stimulus) to the remote location; or transmit within thepipe for detection by an intelligent unit traversing the pipe. As afurther alternative, a tagged element may merely contain an electricallyconductive loop which is sensed by an intelligent unit traversing thepipe and/or by external sensors, in a technical sense rather in themanner of security tags used in departmental stores and the like.

[0078] In general, the tagging technology used may be active (requiringan on-board power source) or passive (not requiring an on-board powersource). Moreover, any convenient signalling technology may be adopted,depending upon various criteria such as the fluid flowing in the pipe,the materials, dimensions and condition of the pipework, the environmentsurrounding the pipework and so on. Candidate technologies include thetransmission and/or reception of electrical, electronic, magnetic,electromagnetic, optical, vibrational, acoustic, or ultrasonic signalsand chemical or radioactive markers.

[0079] Where passive tagging is employed, it is preferred that anintelligent unit (sometimes called a pipeline PIG) is caused to traversethe pipe and is provided with sensors to detect the presence of thepassive tag. Once detected, this information is correlated with thePIG's distance travelled and/or other information that provides anaccurate position for the leak. The positional information may betransmitted by the PIG in real time to an external location where it canbe detected and the information used straight away, or stored on the PIGfor later analysis.

[0080] If the tag is active, it may be used by itself to transmitsignals via any convenient medium, such as along or through the pipewall, though the fluid in the pipe, and/or through the environmentsurrounding the pipework. If pulsed signals are transmitted, timingprocesses can be used to evaluate distance. Any information transmittedexternally of the pipework may be routed to a remote receiver viatransponders supported on, or close to, the pipework, or they may bepicked up directly by detector/receivers placed close to the pipework.In the case of buried pipework, detector/receivers may be lowered intotest bores made close to a suspected leak site.

1. A method of duct leakage control wherein a sheet-like sealing elementis introduced into the duct and is automatically drawn or otherwiseguided to the locality of a leak and is caused, by reason of a pressuredifferential at that locality and attributable to the leak, to beattracted to the leak, thereby to stem or seal the leak.
 2. A methodaccording to claim 1 wherein the sealing element is towed towards theknown position of a leak and released at the leak.
 3. A method accordingto claim 2 wherein a vehicle towing the sealing element is guidedtowards the leak from a location adjacent to the leak.
 4. A methodaccording to claim 2 wherein a vehicle towing the sealing element isguided towards the leak from a remote location.
 5. A method according toclaim 1 wherein said element is conveyed toward said leak by motion of afluid in said duct and once in the vicinity of said leak is drawnthereinto by differential pressure attributable to the leak. 6.Apparatus for duct leakage control comprising a sheet-like sealingelement disposed in the duct and capable of being automatically drawn orotherwise guided to the locality of a leak and is caused, by reason of apressure differential at that locality and attributable to the leak, tomove into and stem or seal the leak.
 7. Apparatus according to claim 6wherein the sealing element comprises a plurality of individual members,each capable of being carried by a flow of fluid along the duct. 8.Apparatus according to claim 7 wherein different members of said sealingelement exhibit differing buoyancies.
 9. Apparatus according to claim 7or claim 8 wherein different members of said sealing element exhibitdiffering shapes.
 10. Apparatus according to claim 6 including sensormeans used to locate the leak, and wherein said sensor means is used totransport or guide the sealing member to the leak.
 11. Apparatusaccording to claim 10 wherein the sealing member takes the form of anopen-ended frustoconical sheet, somewhat like a wind sock in appearance,towed by the sensor means or otherwise guided by the sensor to thevicinity of the leak, whereupon the sheet is drawn towards the leak bythe pressure differential thereacross.
 12. Apparatus according to any ofclaims 6 to 11 inclusive wherein the sealing member supports, in theform of a coating or otherwise, a medium capable, when forced intocontact with the duct, of adhering strongly thereto, thereby anchoringthe sealing member firmly in place across the leaking area of the ductwall and sealing the leak.
 13. Apparatus according to claim 12 whereinsaid medium comprises a bonding agent.
 14. Apparatus according to claim13 wherein the bonding agent is contained in microcapsules which burstin response to the pressure imparted to the sealing element when it isin position to seal the leak.
 15. Apparatus according to claim 11further including pressure sensitive release means, incorporated in thetowing link, which enable the towing vehicle to leave the sealingelement behind at the site of the leak and proceed alone to a suitablecollection point.
 16. Apparatus according to any of claims 6 to 15wherein the sealing element is provided with a location device capable(either by, itself or in co-operation with another element or component)of providing a signal indicative of the location of the sealing means.17. Apparatus according to claim 16 wherein the location devicecomprises a passive device including a loop of electrically conductivematerial, the proximity of which can be sensed by a mobile intelligentunit inside the pipe or a suitable external pick-up.
 18. Apparatusaccording to claim 16 wherein the location device comprises an activedevice such as an infra-red, acoustic, radio or optical senderconfigured to relay signals either directly to the environment outsideof the pipe or to a mobile intelligent unit inside the pipe. 19.Apparatus according to any of claims 6 to 18 wherein two or more sealingdevices of equal or predetermined relative buoyancy are linked together;the linked devices being encouraged to flow at a selected positionwithin the pipe, depending in part upon the length of the link. 20.Apparatus according to claim 19 wherein the linking connection isconstructed to dissolve or change in length during exposure, over aprescribed period of time, to the fluid content of the pipe. 21.Apparatus for leakage control substantially as herein described withreference to the accompanying drawings.
 22. A method of leakage controlsubstantially as herein described with reference to the accompanyingdrawings.